1. Field of the Invention
The present invention relates to a cable gripping block for linear winch applications of the type comprising a base with a pair of mirror image bearing blocks affixed to its top surface. The bearing blocks have matching inclined surfaces that increasingly converge at one end of the base. A pair of mating clamping blocks are mounted between the bearing blocks so that they are relatively movable longitudinally to clamp a cable or wire rope when moved in one direction and to release the cable or wire rope when moved in the opposite direction.
2. Description of the Prior Art
Cable gripping blocks are well known apparatus for clamping a cable, rope or bar for elongated traction or pulling, for example, moving a heavy load horizontally or vertically. The usual arrangement for cable gripping blocks can be used to change the tension on suspension bridge cables, overhead cables on tramways and any application where the free end of a cable is clamped for traction or pulling.
In the prior art, U.S. Pat. No. 4,569,507, a traction block is disclosed which includes an elongated frame having a U-shaped cross section having a flat web and two laterally spaced apart flanges perpendicular to the web, two clamping blocks of elongated shape, which are mounted for movement between the lateral flanges of the frame so that a relative longitude movement between each movable clamping block and the adjacent flange of the frame causes a transverse movement of the respective movable clamping block, and a flat cover plate parallel to the web of the frame and fixed removable thereto. The lateral flanges of the frame are provided with tenons which project from their end faces facing towards the cover plate and which are tightly fitted in corresponding cavities in the cover plate.
The tenons absorb transverse forces usually absorbed by the stud bolts. FIGS. 2 and 3 of U.S. Pat. No. 4,569,507 shows a hinged cover plate 8 with cavities 16 which correspond to tenons 15. There are a number of tenons on each bearing block to absorb the transverse forces exerted by the clamping blocks 5. To secure the cover plate 8 against the tenons, there are fastening bolts 12.
The tenons shown in FIGS. 3 and 4 are tapered, which is necessary since the cover plate is pivotally hinged to the frame. The use of tapered tenons and frusto conical cavities allow the arcuate movement of the cover plate to interfit the tenons in the cavities without binding. However, the tapered tenons and cavities develope a force equal to the formula, EQU F.sub.1 =(cos(L)/2
where
F.sub.1 =the wedging force resisted by one lateral spaced apart flange PA1 L=the angle of the tapered tenon and cavities
measured parallel to the axis of the tenons
and cavities.
This results in a very large and possibly dangerous force which must be resisted by the fastening bolts. The resulting force resisted by the fastening bolts can be calculated to about 28.8 tons, using the data from the patent and measuring the angle of the tenons and cavities. In order to compensate for these possible dangerous forces, the number, size and American Standard Testing Material No. (ASTM) must be calculated, for example using an ASTM number of A307 or A36, the number of bolts and size must correspond as follows: 3/4" diameter 8 bolts; 7/8" diameter--6 bolts, and 1" diameter--4 bolts. In other words, the larger the diameter, the fewer number of bolts required.
In addition to the high tension loads in the fastening bolts, the cover plate may be subjected to a bending force along with the tension load. This bending force is caused by the prying action of the tenons multiplied by the distance between the tenons and the fastening bolts. A secondary bending force may also be present in the cover plate acting between the fastening bolts.